ggplot2 is an R package by Hadley Wickham that lets you make beautiful graphics in R (relatively) easily.
It's part of the Hadleyverse, which I recommend everyone get to know. It includes the packages dplyr, reshape2, stringr, lubridate… and many more.
The name ggplot2 refers to The Grammar of Graphics (a famous book by Leland Wilkinson about data visualization theory).